Various amplifier applications are known in which low noise and high linearity performance are required. Examples of such applications include low noise amplifiers (LNAs), variable gain amplifiers (VGAs), transmit/receive (T/R) modules, power amplifiers, transimpedance amplifiers, and optical receivers used in various products. Such products can include wireless communications, cellular telephony, fiber optic data links, portable electronics.
Many circuits, particularly communication system circuits, require devices for controlling signal level. Variable gain and automatic gain control amplifiers and attenuators are typically used for this purpose.
A problem in reception in mobile communication applications is that the strength of received electromagnetic waves varies over a broad range. In light of this problem, the receiver usually includes an AGC circuit. For the received signal strength indicator (RSSI) to work properly the gain levels must be accurately defined.
A typical conventional variable gain low noise amplifier is implemented utilizing a dual gate MOS FET (Metal Oxide Semiconductor Field Effect Transistor) as a transconductance amplifier. An input signal is applied to a first gate of the FET via an impedance matching circuit. The amplified signal is fed out from the FET via a matching circuit. A DC control voltage is applied to a second gate of the FET in order to determine the gain of the FET. However, in this type of amplifier, the gain control characteristics are non-linear. Additionally the signal gain linearity increase due to reduced gain is often insufficient.
In wireless products there is a need for multiband communications. That is, where the same wireless protocol can be used in varying parts of the RF spectrum. Presently, multi-band enabled wireless products require multiple LNAs (typically 1 per band). The typical LNA for a wireless product is implemented on Gallium-Arsenide (GaAs) integrated circuits (ICs). Multiple LNAs implemented in this manner require a relatively large amount of area on a radio frequency (RF) IC. The IC real estate required for implementation tends to exceed the specification for the implementation.